Some conventional data transmission apparatuses convert digital data into signal levels of electric signals or optical signals to be transmitted. The transmission rates have been increased through the years and, recently, some apparatuses transmit large amounts of data, such as video signals, at transmission rates of several tens of megabits/sec. The frequencies of these signals are so high that emitted noises cause large problems when these signals are transmitted through copper wires or the like.
For example, when such apparatus is mounted on a motor vehicle or the like, emitted noises may cause malfunctions of other electronic equipment that is mounted on the motor vehicle. Accordingly, there is a need to make the apparatus hardly emit noises when mounted on the vehicles. It is also required that the apparatus can transmit data correctly without being affected by noises emitted from other equipment. Similarly, factory automation machinery or precision machines such as medical devices also require reduction in noise emission and resistance to noise.
The conventional data transmission apparatuses utilize a method in which optical fiber cables are employed in place of the copper wires so as to emit no electromagnetic waves. When the copper wires are employed, the voltage of a transmission signal is suppressed at a lower level to reduce emission of noises. There is also employed a method in which a transmission cable for transmitting signals is covered with another shielded wire to prevent the emitted noises from leaking outside. In the case of low-speed signal transmission, a transmission cable such as a twisted pair cable that is obtained by twisting two transmission wires is employed, and signals having opposite polarities are passed through the respective wires, so that the signals cancel each other out, whereby noises are hardly emitted outside. The twisted pair cable has the advantage in having a simple structure and it can be manufactured without great difficulty and accordingly at a reduced cost, while noise emission cannot be reduced satisfactorily at high-speed transmission.
In addition, the digital transmission requires communications with higher reliability. One of factors that reduce the reliability in the digital transmission is that when the transmission signal constantly takes the same signal level, synchronization of symbol timing cannot be obtained at the receiving end.
Conventionally, in order to improve the reliability of receipt, the transmission signal has been processed so that it does not keep on taking the same level. One of the methods for processing the transmission signal is scrambling. The scrambling is a method by which random numbers are added to digital data to be transmitted, thereby to prevent the transmission signal from successively taking the same signal level even when digital data to be transmitted successively take the same value. In the case of binary transmission in which data are transmitted by two values, the data are coded according to the bi-phase mark method, thereby to prevent the same signal level from successively appearing.
The bi-phase mark coding method is employed as a standard transmission method when digital data of audio data are transmitted. FIG. 13 is a diagram for explaining the bi-phase mark coding method. According to the bi-phase mark coding method, depending on whether the immediately preceding symbol is 1 or 0, the next data to be transmitted is coded differently, thereby converting 1-bit data to be transmitted into a 2-bit symbol. Accordingly, a signal sequence that is coded as shown in FIG. 13 is assured that it never takes the same signal level successively three or more times. Thus, the symbol timing of the transmitted data can be detected on the receiving end, whereby the data can be reproduced correctly.
The data transmission apparatus employing optical fibers emits no noise, while it requires expensive elements such as light-to-electricity converters or fiber couplers with less optical loss. In addition, the optical fiber has a problem in its strength, such as limitation in the bend angle of the cable, so that the application range thereof is limited.
Further, according to the method in which the copper signal cable is covered with a shielded wire, some noises are eliminated by the shielding effect, while the shielded wire between the transmitting and receiving ends must be grounded sufficiently to provide effective shielding, and the prices of connectors, cables, or the like for that purpose get higher.
Furthermore, according to the method in which signals having opposite polarities are passed through a twisted pair cable, when the signals to be transmitted include higher frequency components, the signals which flow through two transmission wires of the cable do not always cancel each other out due to slight asymmetry between the two transmission wires, whereby noises occur unfavorably, so that a sufficient reduction in noises cannot be obtained in the case of high-speed data transmission.
Thus, the digital signal to be transmitted is conventionally converted into a rectangular-wave signal having the corresponding signal level, and then higher frequency components are eliminated by means of a low-pass filter utilizing a resistor, a coil, a capacitor, or the like, thereby to reduce noises. However, it is difficult to give steep high-band cut-off characteristics to a filter composed of analog elements, without loosing digital information included in signals being transmitted, and accordingly the noises cannot be eliminated satisfactorily unless the symbol rate of the signal itself is sufficiently low.
In utilizing the scrambling in the data transmission apparatus, when a data pattern to be transmitted matches with a random number sequence employed at the scrambling, the same signal level would successively appear, resulting in that discontinuity of the same signal level cannot always be assured. While the bi-phase mark method assures the discontinuity of the same signal level at the binary transmission, when multi-valued transmission is performed in cases where several bits of data are transmitted at one time, the discontinuity of the same signal level cannot be obtained. In recent years, demands for multi-valued transmission have grown to implement higher-speed digital transmission or more efficient data transmission in a limited band, and the need for a method for more accurate data transmission at the multi-valued transmission has arisen. Further, in order to introduce a new transmission apparatus, replacement of the conventional transmission method or the like should be taken into consideration. More specifically, the new apparatus needs to be able to transmit data of the conventional transmission format without problems and, in the case of audio data for example, it is preferable that it can also transmit bi-phase mark data accurately.
Further, like in a case where the data transmission apparatus is mounted on a motor vehicle or the like, when the apparatus is in such environments that the ground levels of connected devices greatly differ from each other or the fluctuations of the voltage are considerable, it is difficult to correctly transmit the voltage level at the transmitting end to the receiving end. Accordingly, phase modulation or the like is conventionally employed to enable data reproduction even when the absolute voltage cannot be detected accurately between the transmission end and the receiving end. However, a modulation method utilizing a specific carrier frequency unfavorably requires a frequency band that is twice as large as the frequency band of the baseband method that does not utilize the modulation.
Besides, in the data communication on motor vehicles, the amount of electromagnetic waves emitted from the transmission signal is limited so that the electromagnetic waves do not cause malfunctions of other equipment. One of International Standards concerning electromagnetic wave noises emitted from the equipment or communication wires on the motor vehicle is CISPR25. CISPR25 defines a limitation value of emitted noises for each frequency and, particularly, there are strict limitations on signals having frequencies of 30 MHz or higher. Therefore, it is desirable that data should be transmitted in a frequency band of 30 MHz or lower, in which countermeasures against the electromagnetic waves, such as shielding the signal lines to reduce noises, can be taken without great difficulty. In order to transmit data efficiently in this frequency band, a data transmission method that is resistant to voltage fluctuations is needed also when the multi-valued transmission is performed without using modulation.
The present invention is made to solve the above-mentioned problems, and has for its object to provide a digital data transmission apparatus that emits few noises and has a higher resistance to noises, using inexpensive cables such as twisted pair cable, in data transmission at high speeds such as above 20 Mbps, and a transmission line coding method and decoding method in which the same signal level will not successively appear also at the multi-valued transmission.